1. Field of the Invention
The present invention relates in general to a reflective illuminating optical system, more particularly, to a reflective illuminating optical system in which R, G, and B signals are reflected by wire grid type PBS (Polarized Beam Splitters) and incident on a projection lens, thereby suppressing astigmatism and improving illuminating efficiency.
2. Discussion of the Background Art
Display devices are becoming slimmer, lighter and have a large screen. Especially, a large screen display device is an ongoing subject in current display technologies. Projection TVs are typical examples of the large screen display device.
Projection TVs are largely classified into CRT (Cathode Ray Tube) projection TVs and LCD (Liquid Crystal Display) projection TVs. The LCD projection TV consists of a transmissive LCD-based system or reflective LCoS (Liquid Crystal on Silicon)-based system.
Particularly, the reflective LCoS, compared to the transmissive LCD, can be manufactured at low cost.
With reference to FIGS. 1 to 4, a related art projection system and illuminating system will now be discussed.
FIGS. 1 to 4 are schematic diagrams of a related art 3-panel reflective LCoS illuminating system.
As an example of an illuminating system for the related art reflective LCoS-based projection TV, FIG. 1 illustrates a reflective illuminating system with 3 PBSs (Polarized Beam Splitters). As shown in FIG. 1, a light emitted from a lamp 1 passes through a first dichroic mirror 2 via a condensing lens, where the first dichroic mirror 2 reflects red (R) and green (G) lights and transmits blue (B) light.
The reflected R and G lights pass through a second dichroic mirror 3 that reflects the G light and transmits the R light. After a transmission procedure, the R light is incident on first, second, and third PBSs 4a, 4b, and 4c in front of R, G, and B LCoS panels.
The incident R, G, and B lights on the respective 1st, 2nd, and 3rd PBSs 4a, 4b, and 4c are reflected by the PBSs, and are incident upon a first, second, and third LCoS panels 5a, 5b, 5c, respectively. When the R, G, and B lights undergoes a phase change on the 1st, 2nd, and 3rd LCoS panels 5a, 5b, and 5c, and are reflected by the LCoS panels. Then the reflected R, G, and B lights pass through the 1st, 2nd, and 3rd PBSs 4a, 4b, and 4c. 
Those transmitted R, G, and B lights are combined in an X-prism 6, and incident on a projection lens.
To be short, the above-described reflective illuminating system with 3 PBSs has a three-step process for guiding lights: a first step by the lamp 1, and the first dichroic mirror 2, a second step by the second dichroic mirror 3, the second LCoS panel 5b and the second PBS 4b, and a third step by the first and third LCoS panels 5a and 5c, the X-prism 6 and the first and third PBSs 4a and 4c. Because of this structure, the depth of the system is increased.
Moreover, the related art reflective LCoS illuminating system requires a number of elements, e.g., two dichroic mirrors, one mirror, a relay lens for correcting optical path differences of R, G, and B lights, three PBSs, and one X-prism and so forth.
Another example of a related art 3-panel reflective LCoS illuminating system in color quad system of FIG. 2 employs a color selector instead of the relay system.
The illuminating system of FIG. 2 uses a color selector to resolve the optical path differences of R, G, and B lights. That is, when a light emitted from a lamp 7 passes through a first color selector 8a, only a B light wave is changed to a S-wave (Secondary wave), and R and G light waves are outputted as P-waves (Primary waves).
When the light passes through a first PBS 9a, the S-wave is reflected and the P-waves are transmitted. The B light arrives at a second PBS 9b in front of an LCoS panel.
The B light is reflected by the second PBS 9b again, and incident on a third LCoS panel 10c. As the B light is reflected by the third LCoS panel 10c, the B light undergoes a phase change and passes through the second PBS 9b. Afterwards, the transmitted B light is incident on a fourth PBS 9d via a fourth color selector 8d. 
On the other hand, the R and G lights are incident on a third PBS 9c through a second color selector 8b, the G light wave as a S-wave and the R light wave as a P-wave. The 3rd PBS 9c reflects the G light and lets the R light pass through. Then, the G light is incident on a first LCoS panel 10a, and the R light is incident on a second LCoS panel 10b. 
The incident G and R lights undergo a phase change at the 1st and 2nd LCoS panels 10a and 10b, and are incident again on the 3rd PBS 9c where the G and R lights are combined. By a third color selector 8c, the polarization states of the G and R lights become equal, and the G and R lights in the same polarization state are incident on the 4th PBS 9d. 
When the R G, and B lights arrive at the 4th PBS 9d, the lights are combined at the 4th PBS 9d (i.e. the PBS usually performs either P/S separation or composition) and eventually incident on a projection lens.
Therefore, the 3-panel reflective LCoS illuminating system in color quad system has a two-step process for guiding lights, which is relatively simpler than the illuminating system in FIG. 1 where the relay system is provided. However, the illuminating system of FIG. 2 includes four color selectors and four PBSs, so it is not as cost-effective as expected.
In addition, when the PBS performs the P/S separation or composition, the input wave might be in a different polarization state as it is outputted (this phenomenon is called ‘photoelasticity’).
Introduced to solve the problems emerged from the related art illuminating optical systems of FIGS. 1 and 2 is an illuminating system with a wire grid type PBS as shown in FIG. 5. This new illuminating system is cost-effective, solves the photoelasticity problem, and improves illuminating efficiency by using a wide-angle illuminating light.
According to the operational principles of the illuminating system with the wire grid type PBS of FIG. 3, a light emitted from a lamp 11 passes through a first dichroic mirror 12a via a condensing lens. The first dichroic mirror 12a transmits R and G lights and reflects B light.
The transmitted R and G lights pass through a color selector 14, where the G light wave is changed to a S-wave, and the R light wave is changed to a P-wave, and are incident on a second wire grid type PBS 13b. The 2nd wire grid type PBS 13b transmits the R light and reflects the G light. Later, the R light is incident on a first LCoS panel 15a and the G light is incident on a second LCoS panel 15b. 
The G and R lights undergo a phase change at the 1st and 2nd LCoS panels 15a and 15b, and pass through a second dichroic mirror 12b via the 2nd wire grid type PBS 13b, and eventually are incident on a projection lens.
Meanwhile, the B light reflected by the 1st dichroic mirror 12a is reflected by a first wire grid type PBS 13a, and incident on a third LCoS panel 15c. At the 3rd LCoS panel 15c, the B light undergoes a phase change, and passes through the 2nd dichroic mirror 12b via the 1st wire grid type PBS 13a, and eventually is incident on the projection lens.
FIG. 4 illustrates another example of a related art illuminating system with a wire grid type PBS. As shown in FIG. 4, a light emitted from a lamp 16 passes through a first dichroic mirror 17 via a condensing lens. The 1st dichroic mirror 17 reflects R and G lights and transmits B light.
The transmitted B light passes through a second relay lens 18b, a reflective mirror, a third relay lens 18c, and later arrives at a third wire grid type PBS 20c. The B light is reflected by the 3rd wire grid type PBS 20c and incident on a third LCoS panel 21c. 
At the 3rd LCoS panel 20c, the incident B light undergoes a phase change and is reflected again. Finally, the B light is incident on an X-prism 22 via the 3rd wire grid type BPS 20c. 
In the meantime, the R and G lights reflected by the 1st dichroic mirror 17 pass through a first relay lens 18a and is incident on a second dichroic mirror 19 where the R light is transmitted and the G lights is reflected.
The reflected G light is reflected by a second wire grid type PBS 20b and incident on a second LCoS panel 21b. At the 2nd LCoS panel 21b, the G light undergoes a phase change, and passes through the 2nd wire grid type PBS 20b, and eventually is incident on the X-prism 22.
The R light having been transmitted by the 2nd dichroic mirror 19 is reflected by a first wire grid type PBS 20a and incident on a first LCoS panel 21a. At the 1st LCoS panel 21a, the R light undergoes a phase change, and passes through the 1st wire grid type PBS 20a, and eventually is incident on the X-prism 22.
The incident R, G, and B lights are combined at the X-prism 22, and incident on the projection lens 23 later.
The wire grid type PBS in the above-described illuminating system has a homogeneous structure as illustrated in FIG. 5, and is formed on a glass plate.
Here, the size of wire grid type PBS on the glass plate has tens of nanometers.
Although the wire grid type PBS-based illuminating system successfully solved the photoelasticity and cost problems and improves illuminating efficiency, it caused astigmatism.
Astigmatism occurs in case the glass plate is inserted into a focusing lens at an oblique angle. Astigmatism is a phenomenon where a light is defocused at one side because a focal length in the horizontal direction is different from a focal length in the vertical direction.
Astigmatism gets worse especially when the light is reflected by the LCoS panel and then passes through the wire grid PBS.
Referring back to FIG. 3, the G light reflected by the 2nd LCoS panel 15b passes through the 2nd wire grid type PBS 13b, and the B light reflected by the 2nd LCoS panel 15c passes through the 1st wire grid type PBS 13a. 
Also as shown in FIG. 4, the reflected light from the 1st, 2nd, and 4th LCoS panels 21a, 21b, and 21c pass through the 1st, 2nd, and 3rd wire grid type PBSs 20a, 20b, and °c. 
Therefore, when the reflected light from the LCoS panel passes through the wire grid type PBS, astigmatism gets much worse. More details on this phenomenon are provided with reference to FIGS. 6 to 8.
FIG. 6 is a schematic diagram illustrating the layout of a projection lens in case a light passes through a wire grid type PBS; and FIGS. 7 and 8 illustrate the surface of a wave having the same phase.
A simulator is employed to observe aberration characteristics in a case where a wire grid type PBS 50 is inserted at an oblique angle between a screen and an LCoS panel.
FIGS. 7 and 8 show aberration characteristics when, as shown in FIG. 6, the light passes through the wire grid type PBS 50 that is insured at an oblique angle between a projection lens and the LCoS panel.
In other words, astigmatism occurs when the light passes through the wire grid type PBS 50 that is inserted at an oblique angle between a projection lens and the LCoS panel.
The following summarizes several problems of the related art reflective illuminating system.
First of all, the 3-step structure for the optical path in the reflective illuminating system with 3 PBSs of FIG. 1 increases the depth of the system and requires too many elements.
Although the reflective illuminating system in color quad system of FIG. 2 simplified the entire structure, it still includes four color selectors and PBSs, resulting in the high price.
When the PBS performs P/S separation and combination of lights, an input wave might have different components of polarization as the wave is outputted (this phenomenon is called ‘photoelasticity’).
The reflective illuminating system with the wire grid type PBS as shown in FIGS. 3 and 4 solved the problems in relation to photoelasticity, high cost of manufacture, and poor illuminating efficiency. However, when the light passes through the wire grid type PBS, astigmatism occurs.
Astigmatism may be suppressed by manufacturing the wire grid type PBS slimmer or arraying two-wire grid type PBSs in different directions. However, when the wire grid type PBS to be inserted is too thin, the PBS itself gets bent. Also, arraying the PBSs in different directions is not much effective for canceling astigmatism but creates circular shaped spots in large sizes. Further, because the angle between two PBSs is too large, it is impossible to construct the illuminating system overall.